Aluminum gallium nitride etch stop layer for gallium nitride bases devices

ABSTRACT

A semiconductor structure includes a III-nitride substrate with a first side and a second side opposing the first side. The III-nitride substrate is characterized by a first conductivity type and a first dopant concentration. The semiconductor structure also includes a III-nitride epitaxial layer of the first conductivity type coupled to the first surface of the III-nitride substrate, and a first metallic structure electrically coupled to the second surface of the III-nitride substrate. The semiconductor structure further includes an AlGaN epitaxial layer coupled to the III-nitride epitaxial layer of the first conductivity type, and a III-nitride epitaxial structure of a second conductivity type coupled to the AlGaN epitaxial layer. The III-nitride epitaxial structure comprises at least one edge termination structure.

BACKGROUND OF THE INVENTION

Power electronics are widely used in a variety of applications. Powerelectronic devices are commonly used in circuits to modify the form ofelectrical energy, for example, from ac to dc, from one voltage level toanother, or in some other way. Such devices can operate over a widerange of power levels, from milliwatts in mobile devices to hundreds ofmegawatts in a high voltage power transmission system. Despite theprogress made in power electronics, there is a need in the art forimproved electronics systems and methods of operating the same.

SUMMARY OF THE INVENTION

The present invention relates generally to electronic devices. Morespecifically, the present invention relates to methods and systems forusing aluminum gallium nitride layers as etch stop layers ingallium-nitride based devices. Merely by way of example, the inventionhas been applied to the fabrication of edge termination structures foran electronic device and the fabrication of an enhancement mode HEMT.The methods and techniques can be applied to a variety of compoundsemiconductor systems including diodes and transistors.

As described more fully throughout the present specification, an AlGaNlayer grown between two GaN layers is suitable for use as an etch stopin GaN devices. The etch stop is used to determine trench depth throughcontrol of the etch depth. Control of the trench depth enables areproducible manufacturing process for making devices with consistentelectrical characteristics. According to some of the embodimentsdescribed herein, the AlGaN layer is thin (e.g., <10 nm) in comparisonthe thickness of the GaN layers, thereby minimizing any effect on thedevice operation. The AlGaN layer can be doped or undoped depending onthe particular application. Accordingly, the thickness and dopingconcentration associated with the AlGaN etch stop layer are selected toprovide an etch stop layer that results in minimal interference with(i.e., little to no role in) the device functionality or operation.

According to an embodiment of the present invention, a semiconductorstructure is provided. The semiconductor structure includes aIII-nitride substrate with a first side and a second side opposing thefirst side. The III-nitride substrate is characterized by a firstconductivity type and a first dopant concentration. The semiconductorstructure also includes a III-nitride epitaxial layer of the firstconductivity type coupled to the first surface of the III-nitridesubstrate, and a first metallic structure electrically coupled to thesecond surface of the III-nitride substrate. The semiconductor structurefurther includes an AlGaN epitaxial layer coupled to the III-nitrideepitaxial layer of the first conductivity type, and a III-nitrideepitaxial structure of a second conductivity type coupled to the AlGaNepitaxial layer. The III-nitride epitaxial structure comprises at leastone edge termination structure.

According to another embodiment of the present invention, a method offabricating edge termination structures in gallium arsenide (GaN)materials is provided. The method includes providing a n-type GaNsubstrate having a first surface and a second surface, forming an n-typeGaN epitaxial layer coupled to the first surface of the n-type GaNsubstrate, and forming a first metallic structure electrically coupledto the second surface of the n-type GaN substrate. The method furtherincludes forming an AlGaN epitaxial layer coupled to the n-type GaNepitaxial layer, and forming a p-type GaN epitaxial layer coupled to theAlGaN epitaxial layer. Finally, the method includes removing at least aportion of the p-type GaN epitaxial layer to form an exposed portion ofthe AlGaN epitaxial layer and form at least one edge terminationstructure.

According to a specific embodiment of the present invention, aIII-nitride HEMT is provided. The III-nitride HEMT includes a substratecomprising a first n-type III-nitride material, and a drift regioncomprising a second n-type III-nitride material coupled to the substrateand disposed adjacent to the substrate along a vertical direction. TheIII-nitride HEMT also includes an AlGaN barrier layer coupled to thedrift region, a p-type III-nitride epitaxial layer coupled to the AlGaNbarrier layer, a Schottky contact coupled to the p-type III-nitrideepitaxial layer, and a plurality of electrical contacts coupled to theAlGaN drift region.

According to yet another embodiment of the present invention, a methodof processing III-nitride materials is provided. The method includesproviding a III-nitride epitaxial structure including a III-nitridesubstrate, an AlGaN etch stop layer coupled to the III-nitridesubstrate, and a III-nitride epitaxial layer coupled to the AlGaN etchstop layer. The method further includes forming a masking layer onpredetermined portions of the III-nitride epitaxial structure to formexposed regions, exposing the exposed regions of the III-nitrideepitaxial structure to an etchant, and exposing the III-nitrideepitaxial structure to electromagnetic radiation. The methodadditionally includes absorbing a portion of the electromagneticradiation in the III-nitride epitaxial layer, etching at least a portionof the III-nitride epitaxial layer, and terminating the etching in theAlGaN etch stop layer.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentinvention provide methods and systems for fabricated GaN-based devicesincorporating an AlGaN etch stop layer(s) that increase manufacturingyield and reduce device cost. Embodiments of the present invention areapplicable to a wide variety of devices including, without limitation,Schottky diodes with p-GaN guard rings for edge termination, enhancementmode HEMTs, and the like. These and other embodiments of the inventionalong with many of its advantages and features are described in moredetail in conjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are simplified schematic diagrams illustrating a processflow for fabricating an electronic device including an edge terminationstructure using an AlGaN etch stop layer according to an embodiment ofthe present invention;

FIG. 1D illustrates a device structure with a partially removed AlGaNetch stop layer according to an embodiment of the present invention;

FIGS. 2A-2D are simplified schematic diagrams illustrating a processflow illustrating fabrication of a p-GaN enhancement mode high electronmobility transistor (HEMT) including an AlGaN etch stop layer.

FIG. 3 is a simplified flowchart illustrating a method of fabricatingedge termination structures in gallium arsenide (GaN) materialsaccording to an embodiment of the present invention;

FIG. 4 is a simplified flowchart illustrating a method of fabricating aIII-nitride HEMT according to an embodiment of the present invention;

FIG. 5 is a simplified flowchart illustrating a method of processingIII-nitride materials according to an embodiment of the presentinvention; and

FIG. 6 is a simplified graph of illustrating absorption coefficients forAlGaN with different concentrations of Al (up to 50%).

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments of the present invention relate to electronic devices. Morespecifically, the present invention relates to methods and systems forusing aluminum gallium nitride layers as etch stop layers ingallium-nitride based devices. Merely by way of example, the inventionhas been applied to the fabrication of edge termination structures foran electronic device and the fabrication of an enhancement mode HEMT.The methods and techniques can be applied to a variety of compoundsemiconductor systems including diodes and transistors.

Inductively coupled plasma etching processes utilizing gases such as Cl₂and BCl₃ have been used to etch both GaN and AlGaN alloys. The additionof SF₆ to the etchant gas mixture creates a layer of AlF on the AlGaNsurface, which limits the etch rate of AlGaN layers relative to the GaNlayers. The formation characteristics of the AlF layer are dependent onthe aluminum concentration (i.e., mole fraction). Higher pressures,lower dc biases, and higher SF₆/BCl₃ ratios increase the GaN to AlGaNselectivity. The etch rate can be determined by managing both thealuminum mole fraction and SF₆ concentration.

Photo-enhanced chemical etching is an alternative method of selectivelyetching AlGaN/GaN by altering the wavelength of incident light duringetching. Smooth n-GaN surfaces can be obtained at etch rates as high as50 nm/min using a KOH solution and a mercury arc lamp illuminationfiltered at 365 nm. The absorption of light by the GaN results in thecreation of hole electron pairs, which contribute carriers used in theetching process. In contrast with GaN, the absorption edge for AN isnear 200 nm, resulting in negligible absorption at wavelengths between200 nm and 365 nm. FIG. 6 shows the absorption coefficients for AlGaNhaving different Al composition—up to 50% Al. Below the band edge, theabsorption at all compositions is negligible <200 cm-1. Above the bandedge, the absorption coefficient is >80,000 cm-1 depending on the Alcomposition. FIG. 6 is based on findings provided in O. Ambacher, J.Appl. Phys, 31, 2653 (1998), which is hereby incorporated by reference.In some embodiments, an absorption coefficient of the AlGaN etch stoplayer at wavelengths associated with electromagnetic radiation usedduring an etching process is less than 1,000 cm⁻¹.

Therefore, by selecting a wavelength of illumination light less than 365nm, the etching of GaN proceeds, but since the AlGaN is transparent tothe illumination light, the etching process stops when the AlGaN surfaceis reached, thereby resulting in an AlGaN etch stop layer. Thewavelength of light used during the photo-enhanced chemical etchingprocess can be varied depending on the aluminum composition of thestructure.

According to embodiments of the present invention, an advantage of usingAlGaN as an etch stop layer in devices that include trenches is that thedepth of the trench can be made to be dependent on the thickness of theGaN layer. The growth rate on a planar surface can be better controlledthan an etch rate on a disrupted, trench surface. Controlling trenchdepths is only one aspect of controlling a manufacturing process.Varying trench depths in devices that require p-guard rings, forexample, will affect the potential distribution in the guard ringregion. In addition, for many devices, it is desirable to have an etchedsurface that is planar for the subsequent deposition of a dielectricinsulating layer like Si₃N₄, for example. The planarity of the AlGaNlayer provided herein creates such a suitable surface for the depositionof Si₃N₄, other dielectrics, or other appropriate layers.

The thickness of the AlGaN etch stop layer is a predetermined thicknessdepending on the particular application and device design. According toembodiments of the present invention, the thickness of the AlGaN etchstop layer is sufficient to create a barrier to the GaN layer duringetching. In addition, in devices, for example, that utilize verticalcurrent flow through the epitaxial structure, the AlGaN etch stop layershould preferably have an electrical resistivity that does notsubstantially interfere with device performance. According toembodiments of the present invention, the thickness of the AlGaN layerranges from about 3 nm, which is thick enough to provide completecoverage of the underlying GaN layer, to greater thicknesses. Accordingto embodiments of the present invention, the AlGaN layer thickness canrange from about 1 nm to about 30 nm, for example, 10 nm. Otherthicknesses can be utilized depending on the particular application andthe thickness values discussed herein are not intended to limitembodiments of the present invention. The AlGaN etch stop layer can bedoped to form a p-type layer, an n-type layer, or can be undopeddepending on the device application. As will be evident to one of skillin the art, the thickness of the AlGaN layer can be a function of theetch selectivity between GaN and AlGaN, with thinner etch stop layersutilized as the etch selectivity increases.

Embodiments of the present invention provide a process for creating aSchottky barrier diode in GaN with edge termination structures (e.g., afloating guard ring) formed through the etching of an epitaxial layer.FIGS. 1A-1C are simplified schematic diagrams illustrating a processflow for fabricating an electronic device including an edge terminationstructure using an AlGaN etch stop layer according to an embodiment ofthe present invention. Referring to FIG. 1A, a first GaN epitaxial layer112 is formed on a GaN substrate 110 having the same conductivity type.The GaN substrate 110 can be a pseudo-bulk GaN material on which thefirst GaN epitaxial layer 112 is grown. Dopant concentrations (e.g.,doping density) of the GaN substrate 110 can vary, depending on desiredfunctionality. For example, a GaN substrate 100 can have an n+conductivity type, with dopant concentrations ranging from 1×10¹⁷ cm⁻³to 1×10²⁰ cm⁻³. Although the GaN substrate 110 is illustrated asincluding a single material composition, multiple layers can be providedas part of the substrate. Moreover, adhesion, buffer, and other layers(not illustrated) can be utilized during the epitaxial growth process.One of ordinary skill in the art would recognize many variations,modifications, and alternatives.

The properties of the first GaN epitaxial layer 112 can also vary,depending on desired functionality. The first GaN epitaxial layer 112can serve as a drift region for the Schottky diode, and therefore can bea relatively low-doped material. For example, the first GaN epitaxiallayer 112 can have an n− conductivity type, with dopant concentrationsranging from 1×10¹⁴ cm to 1×10¹⁸ cm⁻³. Furthermore, the dopantconcentration can be uniform, or can vary, for example, as a function ofthe thickness of the drift region.

The thickness of the first GaN epitaxial layer 112 can also varysubstantially, depending on the desired functionality. As discussedabove, homoepitaxial growth can enable the first GaN epitaxial layer 112to be grown far thicker than layers formed using conventional methods.In general, in some embodiments, thicknesses can vary between 0.5 μm and100 μm, for example. In other embodiments thicknesses are greater than 5μm. Resulting parallel plane breakdown voltages for the Schottky diode100 can vary depending on the embodiment. Some embodiments provide forbreakdown voltages of at least 100V, 300V, 600V, 1.2 kV, 1.7 kV, 3.3 kV,5.5 kV, 13 kV, or 20 kV.

Referring to FIG. 1A, an n-type GaN substrate is utilized, but thepresent invention is not limited to this particular material. In otherembodiments, substrates with p-type doping are utilized. Additionally,although a GaN substrate is illustrated in FIG. 1A, embodiments of thepresent invention are not limited to GaN substrates. Other III-Vmaterials, in particular, III-nitride materials, are included within thescope of the present invention and can be substituted not only for theillustrated GaN substrate, but also for other GaN-based layers andstructures described herein. As examples, binary III-V (e.g.,III-nitride) materials, ternary III-V (e.g., III-nitride) materials suchas InGaN and AlGaN, quaternary III-nitride materials, such as AlInGaN,doped versions of these materials, and the like are included within thescope of the present invention. Additionally, embodiments can usematerials having an opposite conductivity type to provide devices withdifferent functionality. For example, embodiments provided hereinutilize a heavily doped n-type substrate and p-type edge terminationstructures. However, a device with n-type edge termination structurescan be formed by using materials with opposite conductivity (e.g.,substituting p-type materials for n-type materials, and vice versa) in asimilar manner as will be evident to one of skill in the art.

Although some embodiments are discussed in terms of n-type GaNsubstrates and GaN epitaxial layers, the present invention is notlimited to these particular materials. Thus, although some examplesrelate to the growth of n-type GaN epitaxial layer(s) doped withsilicon, in other embodiments the techniques described herein areapplicable to the growth of highly or lightly doped material, p-typematerial, material doped with dopants in addition to or other thansilicon such as Mg, Ca, Be, Ge, Se, S, O, Te, and the like. Thesubstrates discussed herein can include a single material system ormultiple material systems including composite structures of multiplelayers. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

An AlGaN etch stop layer 114 is coupled to epitaxial layer 112.According to embodiments of the present invention, the AlGaN etch stoplayer has an aluminum mole fraction ranging from about 0.01 to about 0.5(i.e., Al_(0.01)Ga_(0.99)N to Al_(0.5)Ga_(0.5)N). The thickness of theAlGaN etch stop layer can range from about 1 nm to about 30 nm, forexample 10 nm, which provides for a pinhole free layer while reducingthe impact of the etch stop layer on device performance. In someembodiments, the AlGaN epitaxial layer is doped, for example,characterized by a dopant concentration greater than 1×10¹⁷ cm⁻³. Inother embodiments, the AlGaN etch stop layer is undoped or doped atother concentrations.

Referring again to FIG. 1A, an epitaxial layer 120 is coupled to AlGaNetch stop layer 114. In the illustrated embodiment, epitaxial layer 120is a heavily doped p-type layer suitable for use in forming edgetermination structures as well as other electronic uses. The epitaxiallayer 120, from which edge termination structures are eventually formed,can have a conductivity type different than the first GaN epitaxiallayer 112. For instance, if the first GaN epitaxial layer 112 is formedfrom an n-type GaN material, the epitaxial layer 120 will be formed froma p-type GaN material, and vice versa. In some embodiments, theepitaxial layer 120 used to form the edge termination structures is acontinuous regrowth over portions of the first GaN epitaxial layer 112with other portions of the structure, such as regions of othersemiconductor devices, characterized by reduced or no growth as a resultof the presence of a regrowth mask (not shown). One of ordinary skill inthe art would recognize many variations, modifications, andalternatives.

The thickness of the epitaxial layer 120 can vary, depending on theprocess used to form the layer and the device design. In someembodiments, the thickness of the epitaxial layer 120 is between 0.1 μmand 5 μm. In other embodiments, the thickness of the epitaxial layer 120is between 0.3 μm and 1 μm.

The epitaxial layer 120 can be lightly doped in other embodiments, forexample in a range from about 5×10¹⁷ cm⁻³ to about 1×10¹⁹ cm⁻³.Additionally, as with other epitaxial layers, the dopant concentrationof the epitaxial layer 120 can be uniform or non-uniform as a functionof thickness. In some embodiments, the dopant concentration increaseswith thickness, such that the dopant concentration is relatively lownear the AlGaN etch stop layer 114 and increases as the distance fromthe AlGaN etch stop layer 114 increases. Such embodiments provide higherdopant concentrations at the top of the epitaxial layer 120 where metalcontacts can be subsequently formed. Other embodiments utilizeadditional heavily doped contact layers (not shown) to form ohmiccontacts.

FIG. 1B is a simplified cross-sectional diagram illustrating the removalat least a portion of the epitaxial layer 120 to form edge terminationstructures 130/132. As discussed in further detail below, edgetermination structures 130/132 can include any of a variety ofstructures, such as guard rings that circumscribe the Schottky diode toprovide edge termination. Additional discussion related to edgetermination structures is provided in U.S. patent application Ser. No.13/270,606, the disclosure of which is hereby incorporated by referencein its entirety for all purposes.

Additionally, as illustrated in FIG. 1B, the removal (e.g., etch)process is terminated by the AlGaN etch stop layer 114. The presence ofthe etch stop layer provides benefits not available using conventionaltechniques, thereby reducing the control needed over the removalprocess. Inductively-coupled plasma (ICP) etching and/or other GaNetching processes including etching using a BCl₃ or Cl₂, and SF₆ plasmacan be used that are selective for the AlGaN etch stop layer.Additionally, a photo-enhanced chemical etching process can be utilizedas described above.

FIG. 1C illustrates the deposition of an insulating layer 140, forexample, Si₃N₄, in the guard ring region, which provides for electricalinsulation between the elements of the edge termination structures130/132. A deposition and planarization process can be used to forminsulating layer 140.

A Schottky metal structure 150 is formed on the AlGaN etch stop layer114. The Schottky metal structure 150 can be one or more layers of metaland/or alloys to create a Schottky barrier with the AlGaN etch stoplayer 114 and/or other epitaxial layers such as the first GaN epitaxiallayer 112.

As illustrated in FIG. 1C, the Schottky metal structure 150 of theelectronic device 100 can overlap portions of the nearest edgetermination structure 130. The Schottky metal structure 150 can beformed using a variety of techniques, including lift-off and/ordeposition with subsequent etching, which can vary depending on themetals used. Examples of Schottky metals include nickel, palladium,platinum, combinations thereof, of the like.

FIG. 1C also illustrates the formation of an ohmic metal structure 152coupled to the GaN substrate 110. The ohmic metal structure 152 can beone or more layers of metal that serve as a contact for the cathode ofthe Schottky diode. For example, the ohmic metal structure 152 cancomprise a titanium-aluminum (Ti/Al) ohmic metal. Other metals and/oralloys can be used including, but not limited to, aluminum, nickel,gold, combinations thereof, or the like. In some embodiments, anoutermost metal of the ohmic metal structure 152 can include gold,tantalum, tungsten, palladium, silver, or aluminum, combinationsthereof, and the like. The ohmic metal structure 152 can be formed usingany of a variety of methods such as sputtering, evaporation, or thelike.

Although the AlGaN etch stop layer is very thin in some embodiments inorder to reduce or minimize impacts on device functionality, anadditional etching process can be utilized in some embodiments to removethe AlGaN etch stop layer in regions not covered by the unetchedportions of epitaxial layer 120. Thus, the AlGaN etch stop layer can beused as both an etch stop and a protective layer, terminating the etchprocess for epitaxial layer 120 and protecting the underlying epitaxiallayer 112. Since the AlGaN etch stop layer can be thin, an etch that ishighly controllable and reproducible can be used to remove exposedportions of the AlGaN etch stop layer, providing a clean surface ofepitaxial layer 112.

FIG. 1D illustrates an embodiment in which the AlGaN etch stop layer isremoved in the region of the device where the Schottky metal structure150 is formed. The AlGaN etch stop layer remains under the unetchedportions of epitaxial layer 120 as well as under the insulating sections140. In the embodiment illustrated in FIG. 1D, a high quality surface ofthe n-GaN drift layer is provided by the protection enabled by the useof the AlGaN etch stop layer.

In some embodiments, the presence of the AlGaN etch stop layer inregions adjacent to the active device region, i.e., the Schottky metalstructure 150, can increase the breakdown voltage as a result of thehigher aluminum mole fraction of the AlGaN material in comparison to theGaN material. Referring to FIG. 1C, the AlGaN etch stop layer 114positioned under the p-GaN epitaxial layer 120 can enhance the breakdownvoltage of the device in comparison with conventional designs.

Depending on the particular device implementation, the conductivity ofthe AlGaN etch stop layer can be modified as appropriate. The etchselectivity as a function of doping and the thickness of the AlGaN etchstop layer can be related and can, therefore, impact the device design.Tradeoffs between doping levels, the conductivity of the AlGaN etch stoplayer, the thickness of the AlGaN etch stop layer, the aluminum molefraction, and the like can be performed as part of the device design.One of ordinary skill in the art would recognize many variations,modifications, and alternatives.

FIGS. 2A-2D are simplified schematic diagrams illustrating a processflow illustrating fabrication of a p-GaN enhancement mode high electronmobility transistor (HEMT) including an AlGaN etch stop layer. In thedevice illustrated in FIG. 2D, a horizontal (i.e., lateral) p-gated HEMTis provided. The process flow illustrated in FIGS. 2A-2D shares somesimilarities with the process flow illustrated in FIGS. 1A-1C, and,therefore, some redundant description is omitted for purposes ofbrevity.

Referring to FIG. 2A, a substrate 210, an epitaxial layer 212, whichwill provide a drift region of n-type GaN material for the device, andan AlGaN barrier layer 214 are illustrated. The AlGaN barrier layer alsoserves as an etch stop layer as described below. The AlGaN barrier layer214 is coupled to epitaxial layer 212 which isolates the lateral HEMTfrom the substrate 210. As discussed in relation to FIG. 1A, the variousepitaxial layers illustrated in FIG. 2A are provided as examples and arenot intended to limit embodiments of the present invention to theparticular exemplary materials.

A p+ GaN epitaxial layer 220 is grown on the epitaxial stack asillustrated in FIG. 2A in order to provide a p-type material for the p-njunction gated HEMT described below. In embodiments in which a p-typesubstrate is utilized, epitaxial layer 220 can be an n-type layer asappropriate to the underlying epitaxial structure. Referring to FIG. 2B,portions 230 of epitaxial layer 220 are removed (e.g., using aphotolithographic patterning/masking and a BCl₃ or Cl₂ and SF₆ plasmaetching process) to form the gate region 220′. The presence of the AlGaNetch stop layer enables the use of an etching process such as a BCl₃ orCl₂ and SF₆ plasma etching process or a photo-enhanced chemical etchingprocess to pattern the gate region 220′. A blanket deposition of aninsulating material (not shown) (which may also have passivatingproperties) such as silicon nitride, silicon oxide, or the like, can beperformed after removal of portions 230 of the epitaxial layer 220.

As illustrated in FIG. 2C, ohmic metals are deposited and patterned toprovide for electrical contact to the source and drain of the HEMT.Referring to FIG. 2C, metals 260 is a source metal and metal 262 is adrain metal. Metals suitable for use as these contacts include TiAl andother metals that can provide ohmic contacts to AlGaN. The presence ofthe AlGaN etch stop layer 214 enables the use of an etching process suchas a BCl₃ or Cl₂ and SF₆ plasma etching process to be used to patternthe source and drain metals. In other embodiments, a photo-enhancedchemical etching process is utilized. Electrical contacts 260 and 262can be deposited and annealed prior to deposition of Schottky contacts,which are not typically capable of surviving the ohmic contact annealtemperatures. An anneal process for ohmic metals 260 and 262 can beperformed, for example, at a temperature >800° C. for >3 minutes.

Referring to FIG. 2D, ohmic gate metal 264 is deposited and patterned toprovide the gate contact for the HEMT. In some embodiments, the topsurface of gate region 220′ is treated to place it in a conditionsuitable for an ohmic contact to p-GaNand ohmic metal 264 is depositedand patterned using a suitable electrically conductive material.Examples of ohmic metals to p-GaN include nickel, palladium, platinum,combinations thereof, of the like. The geometry of the ohmic contact 264will be a function of the device geometry for the horizontal HEMT.

As illustrated in FIG. 2D, the composition and the doping of the AlGaNetch stop layer can be selected to provide for both etch stopfunctionality during fabrication as well as to support the twodimensional electron gas during operation of the HEMT. As an example,the AlGaN layer can utilize a varying doping concentration, varyingaluminum mole fraction, or the like to provide a layer that is suitablefor etch stop purposes as well as providing the proper electricalperformance for the HEMT. Thus, a layer with varying composition and/ordoping concentration or a series of AlGaN layers with varyingcomposition and/or doping concentration can be utilized in someembodiments of the present invention. As discussed previously, thethickness and the conductivity of the AlGaN etch stop layer(s) can bemodified as appropriate to the particular device design.

Although the HEMT shown in FIG. 2D includes the illustrated layers,other layers, including additional layers, can be utilized asappropriate to the particular device design. Additional descriptionrelated to GaN-based HEMTs is provided in commonly assigned U.S. patentapplication Ser. No. 13/267,552, filed on Oct. 6, 2011, the disclosureof which is hereby incorporated by reference in its entirety. Epitaxiallayers illustrated in this commonly assigned application can be utilizedin the structures described herein to provide desired electricalfunctionality. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

FIG. 3 is a simplified flowchart illustrating a method of fabricatingedge termination structures in gallium arsenide (GaN) materialsaccording to an embodiment of the present invention. The method 300includes providing a III-nitride epitaxial structure (e.g., including ann-type GaN substrate) including an AlGaN epitaxial layer (310). Theepitaxial structure illustrated in FIG. 1A includes an n-typeIII-nitride material substrate, a drift region including a second n-typeIII-nitride material coupled to the substrate and disposed adjacent tothe substrate along a vertical direction, and a p-type III-nitrideepitaxial layer coupled to the AlGaN epitaxial layer. As describedthroughout the present specification, the AlGaN epitaxial layer canprovide etch stop and passivation functionality. The method alsoincludes forming a masking layer on predetermined portions of theIII-nitride epitaxial structure to provide exposed regions (312). Themethod further includes removing a portion of the III-nitride epitaxialstructure above the AlGaN epitaxial layer to form an edge terminationstructure and a Schottky region (314). The masking layer is then removed(316). In some embodiments, the edge termination structure includesmultiple edge termination elements.

The method also includes forming a first metallic structure electricallycoupled to a first portion of the III-nitride epitaxial structure (318)and forming a Schottky metal structure electrically coupled to theSchottky region of the III-nitride epitaxial structure (320). The

Schottky metal structure can be electrically coupled to the at least oneof the edge termination structure elements. In an embodiment the methodcan additionally include forming a metallic field plate (represented bythe portion of Schottky contact 150 overhanging a portion of edgetermination structure 130) coupled to the at least one edge terminationstructure, which can circumscribe a device structure.

It should be appreciated that the specific steps illustrated in FIG. 3provide a particular method of fabricating edge termination structuresin gallium arsenide (GaN) materials according to an embodiment of thepresent invention. Other devices can also be fabricated using themethods described in relation to FIG. 3 as well as throughout thepresent specification. Other sequences of steps may also be performedaccording to alternative embodiments. For example, alternativeembodiments of the present invention may perform the steps outlinedabove in a different order. Moreover, the individual steps illustratedin FIG. 3 may include multiple sub-steps that may be performed invarious sequences as appropriate to the individual step. Furthermore,additional steps may be added or removed depending on the particularapplications. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

FIG. 4 is a simplified flowchart illustrating a method of fabricating aIII-nitride HEMT according to an embodiment of the present invention.The method 400 includes providing a substrate including an AlGaNepitaxial layer (410). The epitaxial structure illustrated in FIG. 2Aalso includes a first n-type III-nitride material substrate, a driftregion including a second n-type III-nitride material coupled to thesubstrate and disposed adjacent to the substrate along a verticaldirection, and a p-type III-nitride epitaxial layer coupled to the AlGaNepitaxial layer. The AlGaN epitaxial layer serves as an etch stop duringmanufacturing of the HEMT as well as a barrier layer of the HEMT. Themethod also includes forming a masking layer on predetermined portionsof the III-nitride epitaxial structure to provide exposed regions (412).As illustrated in FIG. 2B, the exposed regions can define a gate regionfor the HEMT.

A portion of the III-nitride epitaxial structure above the AlGaN layeris removed using an etching process to form the gate region (414). Inthe embodiments described herein, the AlGaN layer can serve as an etchstop for a wet etch process utilizing SF₆ (or other suitable materialthat interacts with the aluminum in the layer to decrease the etch rate)or a photo-enhanced chemical etch process using electromagneticradiation for which the AlGaN layer has low absorption.

The method also includes forming source and drain contacts coupled tothe AlGaN layer (416) and forming a metallic structure coupled to thegate region (418). In an embodiment, the metallic structure comprises anohmic contact coupled to the gate region (e.g., a p-type III-nitrideepitaxial material).

It should be appreciated that the specific steps illustrated in FIG. 4provide a particular method of fabricating a III-nitride HEMT accordingto an embodiment of the present invention. Other devices can also befabricated using the methods described in relation to FIG. 4 as well asthroughout the present specification. Other sequences of steps may alsobe performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 4 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

FIG. 5 is a simplified flowchart illustrating a method of processingIII-nitride materials according to an embodiment of the presentinvention. The method 500 includes providing a III-nitride epitaxialstructure including a III-nitride substrate, an AlGaN etch stop layercoupled to the III-nitride substrate, and a III-nitride epitaxial layercoupled to the AlGaN etch stop layer (510). The III-nitride substratecan be an n-type GaN substrate and the AlGaN etch stop layer can bedoped or undoped. In some implementations, the III-nitride epitaxiallayer is a p-type GaN layer, enabling a p-type layer overlying an n-typelayer to be etched, while not etching a substantial portion of then-type layer. In some embodiments, additional layers are insertedbetween the layers described above.

The method also includes forming a masking layer on predeterminedportions of the III-nitride epitaxial structure to form exposed regions(512), exposing the exposed regions of the III-nitride epitaxialstructure to an etchant such as KOH (514), and exposing the III-nitrideepitaxial structure to electromagnetic radiation (516).

The method further includes absorbing a portion of the electromagneticradiation in the III-nitride epitaxial layer (518), etching at least aportion of the III-nitride epitaxial layer (520) and terminating theetching in or at the AlGaN etch stop layer (522). The etchingselectivity between the AlGaN layer, which does not substantially absorbthe electromagnetic radiation and the GaN layers, which do absorb theelectromagnetic radiation, can vary depending on the absorption edge ofthe AlGaN layer, which is less 365 nm in some embodiments.

It should be appreciated that the specific steps illustrated in FIG. 5provide a particular method of processing III-nitride materialsaccording to an embodiment of the present invention. Other devices canalso be fabricated using the methods described in relation to FIG. 5 aswell as throughout the present specification. Other sequences of stepsmay also be performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 5 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A semiconductor structure comprising: aIII-nitride substrate having a first side and a second side opposing thefirst side, wherein the III-nitride substrate is characterized by afirst conductivity type and a first dopant concentration; a III-nitrideepitaxial layer of the first conductivity type coupled to the firstsurface of the III-nitride substrate; a first metallic structureelectrically coupled to the second surface of the III-nitride substrate;an AlGaN epitaxial layer coupled to the III-nitride epitaxial layer ofthe first conductivity type; and a III-nitride epitaxial structure of asecond conductivity type coupled to the AlGaN epitaxial layer, whereinthe III-nitride epitaxial structure comprises at least one edgetermination structure.
 2. The semiconductor structure of claim 1 furthercomprising a Schottky contact including a second metallic structurecoupled a portion of the AlGaN epitaxial layer.
 3. The semiconductorstructure of claim 2 wherein the second metallic structure is furthercoupled to at least a portion of the at least one edge terminationstructure.
 4. The semiconductor structure of claim 2 wherein the AlGaNepitaxial layer is disposed between the second metallic structure andthe III-nitride epitaxial layer of the first conductivity type.
 5. Thesemiconductor structure of claim 1 wherein the AlGaN epitaxial layer ischaracterized by a thickness less than 30 nm.
 6. The semiconductorstructure of claim 1 wherein the AlGaN epitaxial layer is characterizedby a dopant concentration of greater than 1×10¹⁷ cm⁻³.
 7. Thesemiconductor structure of claim 1 wherein the AlGaN epitaxial layer ischaracterized by an aluminum mole fraction between 0.01 and 0.50.
 8. Thesemiconductor structure of claim 1 further comprising a metallic fieldplate coupled to the at least one edge termination structure.
 9. Thesemiconductor structure of claim 1 wherein the at least one edgetermination structure circumscribes a semiconductor device.
 10. Thesemiconductor structure of claim 9 wherein the semiconductor devicecomprises a Schottky barrier diode.
 11. The semiconductor structure ofclaim 1 wherein: the at least one edge termination structure comprisesthree or more edge termination structures with a plurality of spacingsbetween each of the edge termination structures; a first spacing islocated closer to a semiconductor device than a second spacing; and awidth of the first spacing is smaller than a width of the secondspacing.
 12. A method of fabricating edge termination structures ingallium arsenide (GaN) materials, the method comprising: providing an-type GaN substrate having a first surface and a second surface;forming an n-type GaN epitaxial layer coupled to the first surface ofthe n-type GaN substrate; forming a first metallic structureelectrically coupled to the second surface of the n-type GaN substrate;forming an AlGaN epitaxial layer coupled to the n-type GaN epitaxiallayer; forming a p-type GaN epitaxial layer coupled to the AlGaNepitaxial layer; and removing at least a portion of the p-type GaNepitaxial layer to: form an exposed portion of the AlGaN epitaxiallayer; and form at least one edge termination structure.
 13. The methodof claim 12 wherein the n-type GaN substrate is characterized by a firstn-type dopant concentration and the n-type GaN epitaxial layer ischaracterized by a second n-type dopant concentration less than thefirst n-type dopant concentration.
 14. The method of claim 12 furthercomprising forming a second metallic structure electrically coupled tothe exposed portion of the n-type GaN epitaxial layer to create aSchottky contact.
 15. The method of claim 14 wherein the second metallicstructure is further electrically coupled to the at least one edgetermination structure.
 16. The method of claim 12 wherein removing theat least a portion of the p-type GaN epitaxial layer includes forming ap-type device structure using a remaining portion of the p-type GaNepitaxial layer, the method further comprising forming a second metallicstructure electrically coupled to the p-type device structure.
 17. Themethod of claim 12 further comprising forming a metallic field platecoupled to the at least one edge termination structure.
 18. The methodof claim 12 wherein the at least one edge termination structurecircumscribes a device structure.
 19. A III-nitride HEMT comprising: asubstrate comprising a first n-type III-nitride material; a drift regioncomprising a second n-type III-nitride material coupled to the substrateand disposed adjacent to the substrate along a vertical direction; anAlGaN barrier layer coupled to the drift region; a p-type III-nitrideepitaxial layer coupled to the AlGaN barrier layer; a Schottky contactcoupled to the p-type III-nitride epitaxial layer; and a plurality ofelectrical contacts coupled to the AlGaN drift region.
 20. TheIII-nitride HEMT of claim 19 wherein the drift region has a thicknessbetween 5 μm and 100 μm.
 21. The III-nitride HEMT of claim 19 whereinthe AlGaN barrier layer is characterized by a thickness ranging from 1nm to 30 nm.
 22. The III-nitride HEMT of claim 19 wherein the substratecomprises an n-type GaN substrate.
 23. The III-nitride HEMT of claim 19wherein the plurality of electrical contacts comprise an ohmic sourcecontact and an ohmic drain contact.
 24. The III-nitride HEMT of claim 23wherein the ohmic source contact and the ohmic drain contact areseparated along a horizontal direction.
 25. A method of processingIII-nitride materials, the method comprising: providing a III-nitrideepitaxial structure including: a III-nitride substrate; an AlGaN etchstop layer coupled to the III-nitride substrate; and a III-nitrideepitaxial layer coupled to the AlGaN etch stop layer; forming a maskinglayer on predetermined portions of the III-nitride epitaxial structureto form exposed regions; exposing the exposed regions of the III-nitrideepitaxial structure to an etchant; exposing the III-nitride epitaxialstructure to electromagnetic radiation; absorbing a portion of theelectromagnetic radiation in the III-nitride epitaxial layer; etching atleast a portion of the III-nitride epitaxial layer; and terminating theetching in the AlGaN etch stop layer.
 26. The method of claim 25 whereinthe III-nitride substrate comprises an n-type GaN substrate.
 27. Themethod of claim 25 wherein the AlGaN etch stop layer comprises analuminum mole fraction ranging from about 0.01 to about 0.5.
 28. Themethod of claim 25 wherein the III-nitride epitaxial layer comprises ap-type GaN layer.
 29. The method of claim 25 wherein the etchantcomprises KOH.
 30. The method of claim 25 wherein the electromagneticradiation is characterized by a wavelength less than 365 nm.
 31. Themethod of claim 25 wherein an absorption coefficient of the AlGaN etchstop layer at wavelengths associated with the electromagnetic radiationis less than 1,000 cm⁻¹.